1. Field of the Invention
The present invention relates to a toner and a two-component developer used in the fields of electrophotography and the electrostatic recording, and to an image formation method using the above-mentioned toner and two-component developer, more particularly to a high speed image formation method in which an organic photoconductor belt is used as a latent image bearing member, and a cleaning brush is used as the cleaning means. In addition, the present invention also relates to a toner cartridge holding the above-mentioned toner and an image formation apparatus using the above-mentioned toner.
2. Discussion of Background
In the electrophotographic process, a latent electrostatic image is formed on a photoconductor comprising a photoconductive material, using various means, and the thus formed latent electrostatic image is developed with a toner to a visible toner image, and the developed toner image is then transferred to a sheet of paper when necessary, and fixed thereon with the application of heat and/or pressure thereto, or using a vapor of a solvent, whereby a hard copy can be obtained.
As disclosed in Japanese Laid-Open Patent Application 61-147261, the method of developing the latent electrostatic image is roughly classified into a two-component development system using a toner and a carrier, and a mono-component development system using a toner alone.
In the two-component development system, the toner is mixed and stirred with the carrier so that the toner may become triboelectrically charged to a polarity opposite to that of the carrier. When the toner acquires electrical charges of a polarity opposite to that of the electrostatic image, the toner is deposited on the latent electrostatic image, thereby developing the latent electrostatic image into a visible image.
There are known many development methods depending upon the kind of carrier, for example, magnetic-brush development using iron powder as the carrier; cascade development using a beaded material as the carrier; and fur-brush development using brush fibers. The toner for use in the above-mentioned various development techniques comprises toner particles, each toner particle comprising a binder resin such as a natural resin or synthetic resin, and a coloring agent such as carbon black dispersed in the binder resin.
For instance, to obtain toner particles, a mixture prepared by dispersing a coloring agent in a binder resin such as polystyrene is pulverized until the particle size reaches about 1 to 30 .mu.m. Further, a magnetic toner can be prepared by adding a magnetic material such as magnetite to the components such as the binder resin and the coloring agent.
On the market of the copying and printing apparatus, there is an increasing demand for not only high speed image formation and high quality image formation, but also reduction in size of the apparatus and improvement of durability of the apparatus. In response to such recent demands, the toner, photoconductor, and charge imparting material have been actively developed.
As the means for cleaning the toner particles remaining on the latent image bearing member after image transfer, a blade or fur brush is commonly employed in direct contact with the latent image bearing member. In such an electrophotographic process, the surface of the latent image bearing member, for example, a charge transport layer (CTL) of the photoconductor, is necessarily abraded because the above-mentioned cleaning member and development member are brought into direct contact with the surface of the latent image bearing member. In particular, the photoconductor of the high-speed copying or printing apparatus is required to have such abrasion resistance that can endure large quantities of copies or printings. For the above-mentioned reason, the combination of an organic photoconductor in the form of a flexible belt which has a large available surface area, and a cleaning brush capable of performing relatively moderate cleaning for the photoconductor has become the mainstream in the high-speed copying or printing apparatus. However, even though such combination is adopted, it is not adequate to the high-speed copying or printing apparatus designed to make an enormous volume of copies or printings, for example, more than one million. Namely, still more improved durability is desired with respect to the photoconductor.
In the aspect of the quality of hard copy image, the improvement of preciseness and resolution is strongly desired in recent years. However, the conventional developer has the drawback that since toner particles are selectively subjected to development during making of large quantities of copies and printings for an extended period of time, the particle size distribution of toner particles changes with time in the developer, thereby lowering the resolution of the obtained image.
To obtain a toner image with high preciseness and high resolution, various developers are proposed, as disclosed in Japanese Laid-Open Patent Applications 1-112253, 2-284158 and 7-295283. Each of the above-mentioned developers comprise toner particles with small average particle diameter, and the content of the toner particles with a particle diameter of 5 .mu.m or less, and the particle size distribution are particularly specified.
The toner particles with a particle diameter of 5 .mu.m or less are indispensable for the formation of a toner image with high preciseness and high resolution. It is considered that a latent image can be faithfully and exactly reproduced to obtain a sharp toner image with excellent reproducibility when the toner particles with a particle diameter of 5 .mu.m or less are constantly supplied to the latent image formed on the photoconductor in the development step. On the other hand, the toner particles with a particle diameter of 5 .mu.m or less produce the problem of decrease of the image density. The reason for the decrease in image density is that the intensity of the electric field in the edge portion of a latent image is stronger than that in the center portion thereof, so that the toner deposition amount in the center portion of the latent image becomes less than that in the edge portion when the above-mentioned fine toner particles are employed. However, it is supposed that this problem can be solved by particularly specifying the content ratio by number of toner particles with a particle diameter of more than 5 .mu.m (which will be hereinafter referred to as intermediate toner particles).
The fine toner particles with a particle diameter of 5 .mu.m or less are advantageous for practical use, as previously mentioned, but there exists an optimum content ratio of the above-mentioned fine toner particles.
For instance, in FIG. 1, a toner comprises 17% by number of toner particles with a particle diameter of 5 .mu.m or less. In this case, the content of the toner particles with a particle diameter of 5 .mu.m or less is only 3 wt % of the total weight of the toner particles as shown in FIG. 2. In light of such a small percentage by weight of the fine toner particles, it is doubtful that those fine toner particles can be selectively deposited to the edge portion of a latent image, and the intermediate toner particles can be selectively deposited to the center portion thereof.
In contrast to the above, in FIG. 3, the content ratio by number of toner particles with a particle diameter of 5 .mu.m less is as much as 60%. FIG. 4 is a chart showing the particle size distribution by weight of the same toner shown in FIG. 3. In this case, there is a risk of toner particles being excessively charged under the circumstances of low temperature. The toner particles thus excessively charged are tightly attached to the surface of carrier particles and the surface of the photoconductor. Consequently, the decrease in image density and the fogging are observed in the obtained toner images. In this case, the surface of the photoconductor cannot be perfectly cleaned, and a filming phenomenon takes place on the surface of the photoconductor.
To solve the above-mentioned problem, Japanese Laid-Open Patent Application 4-1773 discloses a toner comprising toner particles with a particle size of 12.7 to 16.0 .mu.m in an amount of 0.1 to 5.0 wt % of the total weight of the toner particles in order to improve the fluidity of toner. In this case, however, it is certain that the obtained fluidity of the above-mentioned toner is inferior to that of the toner comprising 1 to 15% by number of toner particles with a particle size of 5 .mu.m or less. Further, in the case where the content ratio of the large toner particles with a particle size of 12.7 .mu.m or more is increased as disclosed in the above-mentioned application, the image quality of the obtained toner image tends to become uneven.
The fluidity of toner can also be improved by increasing the amount of a fluidity imparting agent. However, the fluidity of toner varies depending upon the contact conditions of the fluidity imparting agent with the surface portions of the toner particles. To be more specific, in the toner containing as much as 60% by number of the toner particles with a particle size of 5 .mu.m or less, the amount of fluidity imparting agent is required to increase 1.5 to 2.0 times the amount thereof necessary for the toner containing 17% by number of the toner particles with a particle size of 5 .mu.m or less in order to obtain substantially the same fluidity. The contamination of the photoconductor and the filming phenomenon on the surface of the photoconductor, and the deterioration of image fixing performance are unavoidable when such a large quantity of fluidity imparting agent is added to the toner particles.
In Japanese Laid-Open Patent Applications 4-124682 and 10-91000, the number of toner particles with a particle size of 5 .mu.m or less is specifically restricted. Although the effects are mentioned in the aforementioned applications when such restriction is established in the preparation of a mono-component developer, there is no description about the particle size distribution of the majority of toner particles dominantly determining the image quality. As a result, a toner image with high resolution cannot be obtained.